From forgetfulness to difficulties concentrating, many people who have long Covid experience “brain fog”. Now researchers say the symptom could be down to the blood-brain barrier becoming leaky.

The barrier controls which substances or materials enter and exit the brain. “It’s all about regulating a balance of material in blood compared to brain,” said Prof Matthew Campbell, co-author of the research at Trinity College Dublin.

“If that is off balance then it can drive changes in neural function and if this happens in brain regions that allow for memory consolidation/storage then it can wreak havoc.”

Writing in the journal Nature Neuroscience, Campbell and colleagues report how they analysed serum and plasma samples from 76 patients who were hospitalised with Covid in March or April 2020, as well 25 people before the pandemic.